Post tensioning concrete reinforcing wires



1965 J. P. KOURKENE POST TENSIONING CONCRETE REINFORCING WIRES Filed July 2, 1962 2 Sheets-Sheet 1 INVENTOR Jack P. Kourkene W,

Dec. 28, 1965 J. P. KOURKENE 3,225,499

POST TENSIONING CONCRETE REINFORCING WIRES Filed July 2, 1962 2 Sheets-Sheet 2 INVENTOR Jock P. Kourkenev 38 I9 Wm m United States Patent 3,225,499 POST TENSIGNING CONCRETE REINFORCING WIRES Jack P. Kourkene, PD. Box 489, Berkeley, Calif. Filed July 2, 1962, Ser. No. 266,644 9 Claims. (Cl. 52-230) This invention relates to a new and improved method and apparatus for post-tensioning concrete reinforcing wires. Reference is made to my co-pending patent application Serial No. 685,285, filed September 20, 1957, now abandoned, of which this application is a continuation-in-part. The method and apparatus are also applicable to prestressing steel structures or structures of other materials.

Post-tensioning concrete reinforcing wires, as commercially practiced, comprises in general the application of tension to wires which pass through concrete structures in various stages of curing. In the more practical methods, heads are formed on the ends of the wires to serve as abutments during application of tension by means of a hydraulic jack or the like. The present invention has important advantages over existing methods and apparatus of this type.

A feature and advantage of the invention is the provision of improved means for anchoring the wires to the concrete structure.

Another advantage of the invention is the improvement of the anchoring for such structures which eliminates the probability of failures of the wires and heads.

Essentially, the present invention provides wires which are formed with heads and are provided with hard, flat surfaces immediately under the heads. In a preferred form, bushings provide such surfaces. An anchor plate is provided with radial slots corresponding in number to the wires, although in one form of the invention hereafter described two or more wires are inserted in each slot. The anchor plate abuts the concrete structure with one or more shims interposed between the inner end of the anchor plate and a bearing plate at the face of the concrete structure. The wires are slipped within the radial slots with the bushings on the outside face of the plate. The hydraulic jack is used to pull the plate outwardly with respect to the concrete structure until the desired tensioning of the wires is accomplished. Thereupon, shims are interposed to hold the wires in tensioned condition. Expanding or regular grout is forced into a socket which has previously been installed at the end of the concrete structure and which, when it has set, anchors the wire in tensioned condition to the structure. Therefore, the anchor plate, bushings and shims may be removed if desired. The foregoing structure and method of application thereof affords numerous advantages over the prior art.

Directing attention first to the use of hardened bushings interposed between the head of each wire and the plate, the advantages thereof include the following: The end of the bushing coming in contact with the button automatically shapes the button so that it seats against the bushing over an extended area. Accordingly, the greater the ress imposed upon the wire, the larger the seat which is created and the larger the area of contact between the button and the bushing. Further, the stress upon the button is automatically directed parallel to the direction of the stress in the wire and thus there is no shear component and no wedging in the seating of the button on the end of the bushing. Further, there is no failure of the head due to brittle failure. Accordingly, the arrangement develops the full strength of the wire in which the button is formed and also develops the full elongation of that wire. Thus, the use of bushings in accordance 3,225,499 Patented Dec. 28, 1965 With the present invention develops tensions of 270,000 psi. and a 7% elongation without introducing a brittle point in the wire, results which are far superior to that in other existing constructions. The foregoing results are obtained with wires presently commercially available, but the invention is applicable to wires stronger than those now available.

A further feature in the use of the bushing is the fact that its diameter is considerably greater than that of the wire and hence permits the wire to be installed in the slot in the anchor plate with a fairly snug fit but applies the compressive stress to the anchor plate in a zone removed from the edge of the slot. This permits, if desired, a less close tolerance in the fit of the wire in the slot because the bushing spans the slot. A symmetrical support is afforded for the bushing, eliminating any tendency of the wire head to snap off or shear off. The use of the bushing also prevents the wire button head from slipping within the slot and forcing its way through the slot to the inner surface of the anchor plate.

The use of bushings considerably reduces the cost of the construction in that hardening the surface of the an chor plate is relatively diflicult and expensive and tends to make the plate brittle. Case hardening the bushings is considerably less complicated and expensive and further enables the manufacturer to use special types of steel for the bushings which are not suitable for the anchor plate.

A further advantage of the use of bushings is the fact that the wire may be cut to required length without the high degree of accuracy required in prior methods and may be installed in place with considerably less accuracy required for the attainment of satisfactory results because they permit full elongation of all the wires of the assembly to be developed and hence safety during the jacking operation is also obtained.

Directing attention now to the advantages of the radially slotted anchor plate:

By using a slotted anchor plate of the type described, the heads may be formed on the wire at a specialized cable fabricating plant remote from the site and connected in place to the anchor plate at a later date than the heads are formed. It permits the anchor plate to be installed after the wires have already been placed in the structure. In fact, the headed wires may be inserted after the concrete has been cast wherever there is a possibility that the wires may be damaged by corrosion such as occurs when the concrete is cured, especially by steam. The anchor plate may be connected to the wire heads before tensioning begins.

Another advantage of the use of the slotted anchor is the fact that the concrete may be partially or completely cured while the wires are being fabricated since the headed wires may be installed within the conduit in the concrete structure after pouring the concrete. Accordingly time of construction is reduced.

A still further advantage of the use of the slotted anchor plate is the fact that it permits economic use of rigid thin wall metal conduits as distinguished from flexible casings to enclose the tensioning wires. Heretofore rigid conduits were not economically feasible (except for less desirable wedging methods not using button-heads) in that wires cannot be inserted therein until immediately before installation in the forms, or after the concrete has set. Rigid conduits cannot be coiled as is possible with flexible conduits and must therefore be shipped to the job site without any wires inserted. Heretofore the wires had to be inserted in rigid conduits, then headed at the job site before installing them in the forms or after the concrete had set. Utilizing slotted anchors, headed wires can be inserted in rigid conduits. Rigid conduits are desirable for a number of reasons. Thus, they tend to protect the wires from corrosion by water leaking into the conduit. The rigid conduits may be oiled with watersoluble oil and thus friction, curvature and wobble efiects of the wires are reduced at the time of tensioning. Flexible casings have ridges which prevent effective oiling or decrease in friction and complete removal of the oil. Also, since rigid ducts do not sag between the points of support as do flexible casings, the resistance to passage of the wires at the time of installation or of tensioning which occurs by reason of the wobble effect or local curvature of the conduit is eliminated. Installing the wires immediately before tensioning reduces any POSSI- bility of the wires rusting between the time the concrete is cast and the time that they are tensioned. Since the wires are grouted immediately after tensioning, the danger of subsequent corrosion is eliminated.

Another advantage of the use of the slotted anchor plate is the fact that the wire ends may be removed after the grout has set and then the anchor plate may be recovered and be re-used as a tool. Further, removal of the anchor plate and its shims reduces the amount of steel which projects from the surface of the concrete structure and hence reduces the amount of labor and materials required to conceal and protect such elements. In addition, the wires need not during manufacturing be cut to an exact predetermined length since they can be subsequently cut to shorter length or bent to improve their bonding characteristics or to fit within the final outline of the structure.

The use of removable slotted anchor plates permits an alternative means of finishing off the structure in that instead of breaking off the heads of the wires after the tensioning is completed, the ends of the wires including the buttons and the bushings can be readily removed from the anchor slots and can be bundled together to form, in effect, a plug which is forced into the socket and adds to the safety of the structure in that it retains by wedging action the prestressing force applied to the wires in the event that the grout should slip along the conical wire perimeter. This final detailing of the wire ends also substantially increases the resistance of the wires against bond slippage.

A further advantage of the invention resides in the provision of the socket and its construction and among these advantages are the following:

The socket transmits a portion of the load to the end plate and can even transmit more than one-half of the total load thereto. The socket also carries some of the stressing load by reason of its lateral bond to the concrete structure in which it is imbedded.

Another advantage of the socket is the fact that it stiffens the bearing plate against bending by the legs of the stressing jacks.

Another advantage of the socket construction is that it encloses the grout during its expansion so as to form a concrete plug under three-dimensional compression which is thus not brittle and which more satisfactorily bonds the grout to the wires.

The socket forms a plug and bonds all of the wires as a group rather than bonding to the individual wires. This condition exists evenwhen the front bearing plate is removed.

Another advantage of the use of the socket is the fact that it forms a recess of sufiicient length to enclose a quantity of grout which insures bonds to the individual wires and further permitsthe wires to be splayed sufliciently so they do not conflict with each other in forming a monolithic group. The plug in effect forms a wedge which secures the wires in place in the event that the grout should crack along the cone of the splayed wires.

Another advantage of the use of the socket is that it forms a recess where the stressing anchor is lodged before tensioning commences, thus reducing the length of the shims and anchorage which must project from the structure. Additionally, the recess is long enough so that the wires are not bent sharply between the end of the casing and the heads of the wires.

Other features and advantages of the invention reside in the use of the end plate heretofore described. Such end plate closes off the socket and keeps the concrete from entering. The end plate transmits some of the prestressing load to the concrete. The use of the plate permits the anchorage assembly to be manufactured and assembled independently of the tendon and to be installed at the time that the concrete forms are installed by carpenters, whereas the tendons may be installed by ironworkers at a later time. In cases where the socket tube remains finally in compression, the end plate need only be connected or tack welded at a few points to the end of the tube and this can be done readily even after installation inside the forms.

An advantage of the end plate hereinafter described is the fact that it reduces the required size of the front bearing plate thereby reducing the overall size of the anchorage assembly and permitting the tendons to be placed closer together in the beam, resulting in further overall savings in construction. For further economy, the end plate can be made the same size as the front bearing p ate.

Other objects of the present invention will become apparent upon reading the following specification and referring to the accompanying drawings in which similar characters of reference represent corresponding parts in each of the several views.

In the drawings:

FIG. l is a vertical sectional view through the completed structure in accordance with this invention.

FIG. 2 is an end elevation.

FIG. 3 is a perspective view of a split shim used in accordance with the invention.

FIG. 4 is an enlarged fragmentary end elevation of a modified construction.

, FIG. 5 is an enlarged fragmentary view showing the button prior to tensioning of the wire.

FIG. 6 is a view similar to FIG. 5 showing the button after stressing. In the form of the invention shown in the accompanymg drawing, the invention is used to post tension a concrete structure indicated generally by reference numeral 11 and partially shown. One end only of structure 11 is illustrated, it being understood that the opposite end is essentially a duplicate. The wires 12 hereinafter described may be tensioned at the end shown in the accompanying drawings or the wires may be tensioned at both ends. Accordingly, it is deemed unnecessary to illustrate the opposite end of the structure.

At the time that the concrete forms (not shown) are prepared and before the concrete which forms structure 11 is poured and vibrated a conduit 13 is installed. Conduit 13 may be a rigid, thin wall member or may be flexible, the rigid construction being preferable. The end of conduit 13 is received within an anchorage assembly indicated generally by reference numeral 16 which is also installed at the time that the concrete forms are prepared. The anchorage assembly 16 consists of an inner plate 17 WhlCh may be a steel member approximately 7" square and A thick having central hole 18 of a diameter equal to that of conduit 13 and a correspondingly dimensioned outer plate 19, spaced apart by a 5" outside diameter tube 21 which is welded to plates 17, 19 at either end to form a rigid structure. Tube 21 may be about 1 feet long and 7 wall thickness. Outer plate 19 has central hole 22 equal to the inside diameter of tube 21. Several wrappings of 2" wide cloth tape 23 are formed around the end of the conduit 13 inside socket assembly 16 to retain conduit 13 inside the socket assembly, or other convenient means may be employed. Tape 23 or other means seals the joint between socket 16 and conduit 13 and prevents conduit 13 from pulling out during conventional vibration.

A plurality of wires 12 is installed in the conduit and passing out through the socket assembly. As has heretofore been mentioned, such wires 12 may be installed before the concrete 11 is poured or they may be installed subsequently, depending upon the conditions at the job site. Each end of wire 12 is formed with a head or button 26 which is upset on the end of wire 12. The upsetting operation is preferably formed at a fabricating shop but may be performed at the job site if desired. The head 26 formed on the end of Wire 12 is hereinafter described in greater detail. Prior to forming heads 26 on the ends of wires 12, hardened bushings 27 are installed, said bushings being approximately /2 in outside diameter and /2" long, and having an inside diameter so that they slide snugly along the Wires 12. The bushings are constructed of material, treated if necessary, that is substantially harder than the wire heads. For tensioning wires commonly used in the art, a bushing anchoring surface hardness of about Rockwell C-55 has been found to give excellent results, although entirely satisfactory results have been obtained with anchoring surface having a Rockwell hardness of the order of C-45 to C65.

Assuming that concrete 11 has been poured and has partially cured over a period of time and that wires 12 have been installed and heads 26 formed thereon if they had not been formed prior to the installation of the wires, circular anchor plate 31 is positioned exteriorly of outside plate 19 or interiorly of socket 16 if the inside diameter of the tube 21 will allow passage of the headed wires 12 and bushings 27 around plate 31. The anchor plate 31 is formed with a plurality of radially inwardly extending slots 32 about /4 long and having a width equal to or slightly greater than the thickness of wire 12 and corresponding in number to the number of wires 12 in the bundle of wires within conduit 13. The inner terminus of each slot 32 is the same distance from the center of the anchor plate 31, which distance is such that wires 12 assume a straight splayed formation when positioned in the slot in about the position shown in FIG. 1. The inner terminii of slots 32 may be round (as in the case of a single slot in FIG. 2) or square. Bushings 27 bear against the portions of plate 31 to either side of slots 32. Slots 32 terminate outward of threaded central hole 33 to provide enough metal to prevent fracture or distortion of plate 31 under tension.

In FIG. 4 two wires 12 are inserted in each slot 32 as is shown by the presence of two heads 26, 26a in each such slot. It will be understood that the number of slots and number of wires per slot is subject to variation. Positioned between the anchor plate 31 and outer plate 19 are shims 36, 37 which are preferably steel plate meeting along a vertical center line 38 of the construction and formed with an arcuate cutout 39 which clears wires 12 and does not conflict therewith. Thus assuming that wires 12 are spaced around a circle of 3% diameter, the radius of cutout 39 of shims 36, 37 is preferably about 1 Shims 36 and 37 are the same in elevation but of different thickness. The number and thickness of the shims depends upon conditions in the specific installation. When anchor plate 31 is stressed away from outer plate 19, one or more pairs of shim 36, 37 are slipped in place. The parting of shims along the vertical line 38 facilitates this operation and forms a vent for allowing the escape of all conduit and socket air during subsequent pressure grouting operation hereinafter described.

In tensioning the wires, hole 33 of anchor plate 31 is engaged by a threaded bolt (not shown) which is coupled to a hydraulic jack supported against outer bearing plate 19. Upon extension of the jack the resulting pull upon anchor plate 31 is transmitted through bushings 27 and heads 26 to each of the wires 12 until anchor plate 31 is extended beyond the surface of the structure, and a proper tension is imparted to each of the wires 12. The rupture or ultimate capacity of such a construction is a minimum of 235,600 pounds for twenty wires of A diameter of present commercial quality. Split shims 36, 37 are then installed in place to hold the anchor plate 31 at the desired distance from the structure. Upon removal of the jack, the assembly remains in place.

The next step in the operation is to fill the interior of conduit 13 and socket 16 with grout 41 by pumping the same therein under high pressure through the opening 33 in anchor plate 31 after the wires 12 have been tensioned. A water to cement weight ratio of about 40% is satisfactory for grout 41. Preferably, an expanding grout is used to improve the bond. An expanding grout may be compounded by addition of gypsum base powder or aluminum powder to the cement or by the use of selfstressing cement compounds. When grout 41 hardens it bonds itself to the socket 16 and to the wires 12, it being desirable that the wires are free of oil or other coating which might inhibit such bonding. The grout 41 becomes a unit or plug in which the wires 12 are firmly imbedded and it is solidly packed behind the shims 36, 37. The grout 41 causes the wires 12 to act as a unit as if they were formed in a solid cable.

Directing attention now to FIGS. 5 and 6, the shape of the head of button 26 is illustrated in detail. In FIG. 5, the button is shown prior to application of stress. In such form the button has a circumferential flat shoulder 51 on its underside transverse to the direction of wire 12 which bears against the end of the bushing 27. Use of shoulder 51 is preferred, but not essential. The Width of shoulder 51 is relatively narrow but it is substantial enough to insure that fiat surface 51 bears against the fiat end 52 of bushing 27. Above shoulder 51, sides 53 of button 26 are bulbous. As has been indicated, if shoulder 51 is eliminated, sides 53 curve outward from the outside diameter of wire 12 itself. The curvature of side 53 is of a radius about equal to the maximum radius of the button transverse to the direction of wire 12. The top 54 of the button is flattened, as illustrated, or rounded. When flat, top 54 has an outside diameter about equal to that of shoulder 51, a form which facilitates upsetting of the button 26 on wire 12.

Upon application of stress the button flares out to the form shown in FIG. 6 or, in other words, the shoulder 51a widens out by outward deformation of the lower part of the sides 53 of the head. If no shoulder 51 existed at the start, shoulder 51a is created by application of stress. Thus the area of the seat between the end 52 of bushing 27 and the shoulder 5101 increases as the tension increases. The change in shape from that shown in FIG. 5 to that shown in FIG. 6 is occasioned by the reason of the facts that the bushing is snug on the Wire, its end 52 is perpendicular to the wire axis and, further, that the hardness of the bushing 27 is greater than the hardness of the head 26. Accordingly, when the end 52 of the bushing comes in contact with shoulder 51 or sides 53 it automatically shapes the button so that it seats against the bushing over an increasingly widening area. It will also be seen that the stress on button 26 is parallel to the direction of the stress on wire 12 and that there is no component of this stress tending to shear off button 26 or to wedge the button into the center hole of bushing 27 or of a hole in the anchor plate.

After the grout 41 has set, shims 36, 37 may be removed, if desired, and likewise plate 31 may be removed. This makes the shims 36, 37 and plate 31 reusable as tools. The projecting ends of wires 12 may be cut or sheared, or otherwise removed, and the structure 11 surfaced to conceal the ends of Wires 12. Bushings 27 may also be reclaimed and used again. It will be understood, however, that plate 31, shims 36, 37 and bushings 27 may be retained in place if desired.

A plurality of wires 12 is illustrated and described. However, in some cases a single wire may be sufficient.

What is claimed is:

1. In a prestressed structure and a stressing, reinforcing wire and anchor structure therefor, the combination comprising a structure to be prestressed, a plurality of wires extending through said structure to be prestressed, each of said wires formed on at least one end with an enlarged head, said head being bulbous and enlarging in diameter outwardly of the outside diameter of said wires, and anchor means having an aperture therethrough engaging each of said heads, each aperture being slightly larger than said wire and less than the outside diameter of said head, said anchor means being slidable relative to said wire and engaging the underside of said head, and tensioning means engaging said anchor means, said anchor means being of substantially greater hardness than said head so that upon tensioning of said wire said head is deformed with a substantially flat shoulder extending radially outwardly from the wire on the underside of said head and in close contact with said apertured means.

2. The combination as in claim 1 in which said apertured means has a hardness in the range of Rockwell C45-C-65.

3. A stressing, reinforcing wire and anchor structure comprising a plurality of wires extending through a structure to be prestressed, each formed on at least one end with an enlarged head, a plurality of bushings disposed at least one to a wire and slidable therealong so as to abut the heads of said wires, said bushings being substantially harder than said wire heads, an anchor plate formed with apertures about the periphery thereof through which the wires extend, each of the apertures slightly larger than the diameter of the wire but narrower than the width of the bushings, the inner end of said bushings engaging the outer face of said anchor plate, means formed on said anchor plate for engaging a tensioning device and for moving said anchor plate relative to said structure to be prestressed to thereby enable stressing of said wires, and means for securing said anchoring plate means so as to maintain said wires in stressed condition.

4. A structure according to claim 3 in which said means for securing said anchor plate comprises at least one shim disposed between said anchor plate and said structure to be prestressed.

5. A structure according to claim 3 in which said means for securing said anchor plate comprises a body of bonding material disposed between said anchor plate and said structure to be prestressed, said body of bonding material being bonded to said reinforcing wires and said anchor plate.

6. A method of post-tensioning construction material with reinforcing wires comprising the steps of providing a structure of said material with an elongated conduit extending therethrough, passing at least one reinforcing wire through said conduit, said wire being provided with an integral bulbous head, anchoring the other end of said wire, positioning an anchoring surface of a hardness substantially greater than the hardness of said head under said head to bear against the underside of said head, stressing said anchoring surface away from the other anchored end of said wire and away from said structure to tension said wire and thereby to form on said head a radially outwardly extending shoulder on the underside thereof in contact with said anchoring surface, and securing said anchoring surface so as to maintain said wire in stressed condition.

7. A method of post-tensioning construction material with reinforcing wires comprising the steps of providing a structure of said material with an elongated conduit extending therethrough, providing said conduit with an enlarged outwardly facing recess at one end, passing at least one reinforcing wire through said conduit and through said recess, said wire being provided with an integral bulbous head, anchoring the other end of said wire, positioning an anchoring surface of a hardness substantially greater than the hardness of said head under said head to bear against the underside of said head, stressing said anchoring surface away from the other anchored end of saidwire and away from said structure to tension said wire and thereby to form on said head a radially outwardly extending shoulder on the underside thereof in contact with said anchoring surface, securing said anchoring surface so as to maintain said wire in stressed condition, and filling said conduit and the recess at the end thereof with a bonding material which surrounds and bonds to said wire and forms a plug in said recess additionally holding said wire in stressed condition.

8. A method as in claim 7 in which said bonding material is of an expanding type.

9. A method of post-tensioning construction material with reinforcing wires comprising the steps of providing a structure of said material with an elongated conduit extending therethrough, forming an enlarged outwardly opening recess in said structure in communication with said conduit, passing a plurality of reinforcing wires through said conduit and through said recess, said wires being provided with integral bulbous heads on at least one end, anchoring the other ends of said wires, positioning at least one anchoring surface of a hardness substantially greater than the hardness of said heads under said heads to bear against the underside of the same, stressing said anchoring surface away from the other anchored ends of said wires and away from said structure to tension said wires and thereby to form on said heads radially outwardly extending shoulders on the underside thereof in contact with said anchoring surface, securing said anchoring surface so as to maintain said wires in stressed condition, filling said conduit and the space around and between said wires within said recess With a bonding material to thereby form a plug additionally holding said wires in stressed condition.

References Cited by the Examiner UNITED STATES PATENTS 289,164 11/1883 Strohm 50-517 1,097,741 5/1914 Voight 16l68 1,979,894 11/1934 Lyons 16168 2,371,882 3/1945 Freyssinet 50135' 2,580,174 12/1951 Henderson 50179 2,728,978 1/1956 Birkenmaier 50135 2,783,024 2/ 1957 Lee 25429 2,867,884 1/1959 Brandt 50128 2,948,995 8/1960 C-ogan 50-128 2,988,794 6/1961 Gutt 50128 3,029,490 4/ 1962 Middendorf 50-428 FOREIGN PATENTS 864,170 3/1961 Great Britain.

333,819 12/1958 Switzerland.

FRANK L. ABBOTT, Primary Examiner.

HENRY C. SUTHERLAND, JACOB L. NACKENOFF,

Examiners. 

3. A STRESSING, REINFORCING WIRE AND ANCHOR STRUCTURE COMPRISING A PLURALITY OF WIRES EXTENDING THROUGH A STRUCTURE TO BE PRESTRESSED, EACH FORMED ON AT LEAST ONE END WITH AN ENLARGED HEAD, A PLURALITY OF BUSHINGS DISPOSED AT LEAST ONE TO A WIRE AND SLIDABLE THEREALONG SO AS TO ABUT THE HEADS OF SAID WIRES, SAID BUSHINGS BEING SUBSTANTIALLY HARDER THAN SAID WIRE HEADS, AN ANCHOR PLATE FORMED WITH APERTURES ABOUT THE PERIPHERY THEREOF THROUGH WHICH THE WIRES EXTEND, EACH OF THE APERTURES SLIGHTLY 